This invention relates generally to the detection of ions in spectrometry, and more particularly to a data acquisition system including methods of operation and apparatus for acquiring and processing data from a time-of-flight mass spectrometer.
The science of mass spectrometry has been proven to be a valuable tool in analytical chemistry. Mass spectrometry is premised on the fact that electrically neutral molecules of a sample can be charged or ionized and their motion controlled by electric and magnetic fields. The response of a charged molecule to magnetic and electric fields is influenced by the mass-to-charge ratio of the ion so that ions of a specific mass-to-charge ratio can be selectively detected.
Mass spectrometers differ from each other primarily in the way in which ions of different mass-to-charge ratios are distinguished from each other. Magnetic sector mass spectrometers separate ions of equal energy by the ions' momentum as they are reflected or dispersed in a magnetic field. Quadrupole mass spectrometers separate ions based upon their rate of acceleration in response to a high frequency radio frequency field in the presence of a direct current field. Ion cyclotrons and ion trap mass spectrometers discriminate ions on the frequency or dimensions of their resonant oscillations in alternating current fields. Time-of-flight mass spectrometers discriminate ions according to their velocity over a fixed distance.
Although relatively straightforward in design, time-of-flight (hereinafter “TOF”) mass spectrometers produce data at a very high rate. Because ions having different mass-to-charge ratios may be present in a single sample, they will strike the ion detector at different times according to their velocity or kinetic energy. The detector output signal comprises a sequence of ion arrival responses which are compressed within a very short time interval, generally less than one-tenth of a microsecond. Within a few milliseconds, all of the ions, including the heaviest, have traveled the length of the typical TOF mass spectrometer and arrived at the detector to produce a spectrum of this sample molecule. Up to as many as one million spectra may be produced for a given sample analyzed. Additionally, these spectra may need to be separated into chronologically ordered sets. The time scale would be on the order of one millisecond.
Scientists often wish to collect data over the entire spectra (i.e., the full mass range) that may be produced by the sample. Past systems have generally operated slowly. Although it is desirable to speed up the data acquisition systems to match the capabilities of a TOF spectrometer, a problem arises in the need to store the data as it is produced. In some circumstances, only a small segment containing certain ionic compounds of all of the data produced by the analysis of a given sample may be of interest. Thus, to reduce the amount of data produced, and to focus in on the ionic compound of interest, it has been proposed to turn the detection circuit on just prior to the predicted arrival time or window of a selected compound. Details of such a system are disclosed in U.S. Pat. No. 5,367,162, owned by the assignee of this invention. This patent also provides a thorough discussion of the prior art, and its disclosure is incorporated herein by reference.
The data acquisition systems disclosed in commonly owned U.S. Pat. Nos. 5,712,480 and 5,981,946 increased the speed of collection and processing data and improved resolution by collecting larger segments of data over a shorter time interval than previously available. These data acquisition systems digitize and temporarily store all of the particular data of interest.
Some systems have been developed that sample the ion detector output signal at a very high rate (i.e., up to 1.0 GHz). However, for such systems to sample at such a high rate, other sacrifices were made, such as the rate at which spectra is generated and reported, the mass range over which the spectra is generated, and/or the mass precision of its measurements. Thus, the prior systems have been subject to various trade-offs when attempting to increase the sampling rate. In addition, in these systems, most of the processing of the spectra occurred in an external PC rather than on the data acquisition board of the mass spectrometer. Typically, the data from the data acquisition board was dumped into a memory for later processing by the external PC. Further, the lack of sufficient memory and the lack of sufficient write speeds of the memory limited the ability to speed up the sampling rate of the ion detection signal.
Because of trade-offs between the spectra generating/reporting rates and the mass range over which the spectra is generated, prior systems have often limited the selected mass range as a function of the selected spectra reporting rate or vice versa. For example, a system that may allow for a spectra reporting rate of 30 spectra/second would allow selection of analysis over a full mass range, but may not allow analysis over a full mass range if one selected a higher spectra reporting rate. Likewise, for a limited mass range, a spectra reporting rate of 45 spectra/second may be selected, but if one wished to select a full mass range, the spectra reporting rates that may be selected would decrease.
Another issue with present data acquisition systems is that they either use an ion detection configuration that is sensitive enough to detect a single ion appearing in only one of many consecutive transients (merely counting the number of voltage spikes that are above a fixed threshold) or they use an ion detection configuration that is capable of detecting and quantifying the number of ions simultaneously striking the ion detector (using an analog-to-digital converter). However, such systems have not had sufficient dynamic range and sensitivity to do both.